VIS: default LinePlot rotation of 0

Closes https://github.com/pydata/pandas/issues/8150

```python
In [1]: s = pd.Series([1,2,3])

In [2]: s.plot()
Out[2]: <matplotlib.axes._subplots.AxesSubplot at 0x10da02d68>
```

![ex](https://cloud.githubusercontent.com/assets/1312546/4310965/f43fd70c-3eaa-11e4-9e2a-aed38bd73ae3.png)


No release notes since this is just changing it back to how it was for the last release.